Canvas_tutoriál/Použítí_obrázků

Importování obrázků

Importování obrázků je v podstatě dvou-krokový proces:

• V prvé řadě potřebujeme reference k JavaScriptovému Image objektu nebo jinému canvas prvku jako zdroj. Není vhodné používat obrázky prostým uvedením jejich URL/cesty k nim.
• Potom vykreslíme obrázek do canvas použitím funkce drawImage.

Podívejme se na první krok. Zde jsou v zásadě k dispozici 4 možnosti:

Používání obrázků, které jsou na stejné stránce

Můžeme přistupovat ke všem obrázkům na stránce pomocí kolekce document.images, metody document.getElementsByTagName nebo pokud známe atribut ID obrázku, tak metodou document.getElementById.

Použití dalších prvků plátna

Just as with normal images we access other canvas elements using either the document.getElementsByTagName method or the document.getElementById method. Make sure you've drawn something to the source canvas before using it in your target canvas.

One of the more practical uses of this would be to use a second canvas element as a thumbnail view of the other larger canvas.

Creating an image from scratch

Another option is to create new Image objects in our script. The main disadvantage of this approach is that if we don't want our script to halt in the middle because it needs to wait for an image to load, we need some form of image preloading.

Basically to create a new image object we do this:

var obrazek = new Image();      // Vytvoří nový objekt Image
obrazek.src = 'mujObrazek.png'; // Nastaví cestu zdroje

When this script gets executed, the image starts loading. If loading isn't finished when a drawImage statement gets executed, the script halts until the image is finished loading. If you don't want this to happen, use an onload event handler:

var obrazek = new Image();      // Vytvoří nový objekt Image
obrazek.src = 'mujObrazek.png'; // Nastaví cestu zdroje
obrazek.onload = function(){
  // spustí se zde příkaz drawImage
}

If you're only using one external image this can be a good approach but once you need to track more than one we need to resort to something more cunning. It's beyond the scope of this tutorial to look at image preloading tactics but you can check out JavaScript Image Preloader for a complete solution.

Vložení obrázku pomocí data: url

Another possible way to include images is via the data: url. Data urls allow you to completely define an image as a Base64 encoded string of characters directly in your code. One advantage of data urls is that the resulting image is available immediately without another round trip to the server. ( Another advantage is that it is then possible to encapsulate in one file all of your CSS, Javascript, HTML, and images, making it more portable to other locations. ) Some disadvantages of this method are that your image is not cached, and for larger images the encoded url can become quite long:

var img_src = 'data:image/gif;base64,R0lGODlhCwALAIAAAAAA3pn/ZiH5BAEAAAEALAAAAAALAAsAAAIUhA+hkcuO4lmNVindo7qyrIXiGBYAOw==';

Metoda drawImage()

Once we have a reference to our source image object we can use the drawImage method to render it to the canvas. As we we'll see later the drawImage method is overloaded and has three different variants. In it's most basic form it looks like this.

Where obrazek is a reference to our image or canvas object. x and y form the coordinate on the target canvas where our image should be placed.

Příklad metody drawImage()

In the following example I will be using an external image as the backdrop of a small line graph. Using backdrops can make your script considerably smaller because we don't need to draw an elaborate background. I'm only using one image here so I use the image object's onload event handler to execute the drawing statements. The drawImage method places the backdrop on the coordinate (0,0) which is the top left corner of the canvas.

Zobrazit tento příklad

  function kresli() {
    var ctx = document.getElementById('canvas').getContext('2d');
    var img = new Image();
    img.src = 'backdrop.png';
    img.onload = function(){
      ctx.drawImage(img,0,0);
      ctx.beginPath();
      ctx.moveTo(30,96);
      ctx.lineTo(70,66);
      ctx.lineTo(103,76);
      ctx.lineTo(170,15);
      ctx.stroke();
    }
  }

Scaling

The second variant of the drawImage method adds two new parameters and it allows us to place scaled images on the canvas.

Where sirka and vyska is the image's size on the target canvas.

Příklad 2 metody drawImage()

In this example I'm going to use an image as a wallpaper and repeat it several times on the canvas. This is done simply by looping and placing the scaled images at different positions. In the code below the first for loops through the rows the second for loop the columns. The image is scaled one third of its original size which is 50x38 pixels. We'll see how this could also have been achieved, by creating a custom pattern, later in this tutorial.

Zobrazit tento příklad

  function kresli() {
    var ctx = document.getElementById('platno').getContext('2d');
    var img = new Image();
    img.src = 'images/rhino.jpg';
    img.onload = function(){
      for (i=0;i<4;i++){
        for (j=0;j<3;j++){
          ctx.drawImage(img,j*50,i*38,50,38);
        }
      }
    }
  }

Slicing

The third and last variant of the drawImage method has eight new parameters. We can use this method to slice parts of a source image and draw them to the canvas.

The first parameter obrazek, just as with the other variants, is either a reference to an image object or a reference to a different canvas element. For the other eight parametes it's best to look at the image on the right. The first four parameters define the location and size of the slice on the source image. The last four parameters define the position and size on the destination canvas.

Slicing can be a useful tool when you want to make compositions. You could have all elements in a single image file and use this method to composite a complete drawing. For instance, if you want to make a chart you could have a PNG image containing all the necessary text in a single file and depending on your data could change the scale of your chart without very much diffculty. Another advantage is that you don't need to load every image individually.

Příklad 3 metody drawImage()

In this example I'm going to use the same rhino as we've seen above, but now I'm going to slice its head out and composite it into a picture frame. The image of the picture frame includes a dropshadow which has been saved as a 24-bit PNG image. Because 24-bit PNG images include a full 8-bit alpha channel, unlike GIF and 8-bit PNG images, I can place it onto any background and don't have to worry about a matte color.

I took a different approach to the loading of the images than the example above. I just placed the images directly in my HTML document and used a CSS rule to hide them from view (display:none). I assigned both images an id attribute to make them easier to select. The script itself is very simple. I first draw the sliced and scaled image on the canvas (first drawImage statement), and then place the frame on top (second drawImage statement).

Zobrazit tento příklad

function kresli() {
  var platno = document.getElementById('platno');
  var ctx = canvas.getContext('2d');

  // Draw slice
  ctx.drawImage(document.getElementById('source'),
                33,71,104,124,21,20,87,104);

  // Draw frame
  ctx.drawImage(document.getElementById('frame'),0,0);
}

Příklad umělecké galerie

V posledním příkladu této části, vytvoříme jednoduchou uměleckou galerii. The gallery consists of a table containing several images. When the page is loaded, for each image in the page a canvas element is inserted and a frame is drawn arround it.

In my case, all images have a fixed width and height, and so does the frame that's drawn around it. You could enhance the script so that it uses the image's width and height to make the frame fit perfectly around it.

The code below should be self-explanatory. We loop through the images array and add new canvas elements accordingly. Probably the only thing to note, for those not so familar with the DOM, is the use of the insertBefore method. insertBefore is a method of the parent node (a table cell) of the element (the image) before which we want to insert our new node (the canvas element).

Zobrazit tento příklad

function draw() {

  // Loop through all images
  for (i=0;i<document.images.length;i++){

    // Don't add a canvas for the frame image
    if (document.images[i].getAttribute('id')!='frame'){

      // Create canvas element
      canvas = document.createElement('CANVAS');
      canvas.setAttribute('width',132);
      canvas.setAttribute('height',150);

      // Insert before the image
      document.images[i].parentNode.insertBefore(canvas,document.images[i]);

      ctx = canvas.getContext('2d');

      // Draw image to canvas
      ctx.drawImage(document.images[i],15,20);

      // Add frame
      ctx.drawImage(document.getElementById('frame'),0,0);
    }
  }
}